We have used an immunocytochemical procedure to localize contractile proteins with respect to individual skeletal muscle fibers. We have shown that more than one isoenzyme of myosin can exist in a single muscle and that the component fibers can be identified on the basis of immunological differences in their myosins. This approach will be used to examine the relationships of cytochemical features to physiological properties of the same fibers. The pattern of distribution of isoenzymes of myosin will be analyzed in skeletal muscles of the cat, where physiological characteristics of individual fibers have already been demonstrated. Developmental changes will be analyzed in skeletal muscles of the rat at stages in which we have already determined the major structural characteristics. This study will also include ultrastructural immunocytochemistry. Antibodies against myosin will be used as "markers" to identify reacted and unreacted muscle fibers with the electron microscope. It should therefore be possible to correlate immunocytochemical with ultrastructural features and ultimately with physiological properties.